Port area in relation to net volume,power,tuning..

[anr102990]

 I’m somewhat unsure of optimal port area in relation to a specific woofer, the net volume, power applied, and tuning frequency. My sub is a skar DDX 12 dual 2 wired to one ohm on a skar rp 1500. It’s currently in a box I built for it , 2.35 ft3 net @ 36hz with 37 sq in of port area. It’s a bit peaky in my opinion, drops off really sharp around 33 and below and plays well up to 48 or so and drops off again, I’m sure cabin gain plays a role as well. I’m about to design a new box, tuned lower, with the goal of playing a flatter bandwidth musically from at least 28 to around 50 if possible. I need some of you gurus to help me accomplish this please. I was playing with numbers in win isd up until my laptop took a shit this morning and won’t charge. So, Do I need a little less port area when tuning lower and with a little more net volume? As in increasing the net volume to 2.5-2.6 and decreasing the port area a little? Have heard over and over about the 12-16” and blah blah but I’m looking for optimal volume with a flatter musical response for my specific system. I recently read an article where higher tuning benefits from more port area whereas lower tuning is helped with a little less port area.  Any help will be appreciated. Pic is of my current sub/enclosure @Triticum Agricolam Can you elaborate a little more on how watts per cubic foot relates to port area, specifically slot ports vs aero ports? Would a slot port need more area due to having more surface area from the walls of the port itself? I’m really trying to learn all this and learn it correctly the first time as I get deeper into the real deal specifics of design..

[mothra]

I too was in your position several years back. I just went out and bought Bass Box Pro 6. I noticed that there were a lot of "rules" and you need this or that to sound good or loud, and my favorite "this sub is only for ported enclosure". well I found that most of that was just not the case when I saw the data plotted out. there is nothing magical, it's all science and compromise. I did a class with Andy Wehmeyer and he had even more data to support the science and compromise. when I build boxes, I'm after 2 things. 1 is port velocity between 25-30m/s. 2 is keeping tuning between 28-30hz. I see too many people/professional say tuning should be between 34-38hz b/c of cabin gain. I've heard those boxes and they do sound good but don't get those lows of rap or dub step. I know the 2 things I look at when building are not the most vital but I'm willing to compromise on the other stuff. I know other builders focus on other things and are willing to compromise on my stuff. I get it and those guys build great boxes. It's just another way of doing it.

I'd personally recommend you get Term-Pro or Bass Box Pro so you can see the plots frequency response, port velocity, etc.... to really get a sense of what's going on when you build a box.

[anr102990]

Ok thanks man, and yea I definitely am ready to have a lower tuning with a more efficient enclosure. I had not being able to play all my low low dj Russticals tracks on my new 64gb flash drive lol. Also, my sub is finally broken in well, which I believe will allow a better output on the lower frequencies. On those programs you were telling me about, is the port velocity in m/s dependent upon power applied to the sub? As in more power would result in higher velocities? And thus needing more port area? 

[Triticum Agricolam]

fasfocus00 pretty much nailed it, everything about box design is a compromise is some way.  The trick is figuring out what aspects are most important to you and what areas you are willing to sacrifice a little. 

 

When it comes to port area, its my view that there is no situation where reducing port area provides any direct benefit.  Contrary to what you may hear on the internet, reducing port area is not an effective way to improve cone control or flatten frequency response.  Reducing port area CAN provide some indirect benefit though, since smaller ports take up less space, so by making the port smaller you may be able to increase box net volume, or reduce the overall size of the box.  This is where the tradeoff comes in, we want a large enough port to avoid port noise and compression, but we don't want to the port to take up any more space than it needs to since with car audio we are almost always space constrained.  

 

Like you said, the three biggest factors when it comes to how much port area you need are box net volume, tuning frequency, and input power.  Sub specs play a part as well, but its not as important and it might seem.  Plugging things into modeling software, such as WinISD, TermPro, or BassBoxPro is the best way to figure out how much port area you need.  The port area calculator I made does work well (IMHO), but it will never be as accurate as actual modeling software.  When determining port area I like to keep peak port velocity under 22 m/sec if I can, though that's not always possible so I just do the best I can.  32 m/sec is the absolute upper limit, above that and you start losing a LOT of output to port compression.  

 

As far as slot ports and aero ports go,  aero ports don't benefit from less port area, the difference is you can get away with a bit less port area before bad things start happening with aero ports.  The reason is as you said, aero ports have less internal surface area for a given amount port area.  This only applies when using one or two larger aero ports.  Once you have three or more aero ports a single slot port will often have less internal surface area.  

 

[anr102990]

Thanks guys, I’ll plug and play with the software soon as I can get my laptop running again. I’m sure it will be trial and error of building boxes and finding just what I want, and by that time my goals may end up changing, who knows. Also, I’m leaning towards an aero port design mostly for an easier change of port tuning but also just because I’ve never had/built one. 

[mothra]

port velocity is dependent on power and port area mainly. when you get a velocity over 32 m/s you start to get chuffing noises. lowering power or increasing port area will help reduce the port velocity. I do a lot of builds in SUVs and CUVs with moderate equipment and power. Those guys want that intense loud feeling, which is moving air. What makes people have that "WOW" experience in a loud vehicle is the hair trick. most of they types of builds I get are 2 15s, 3 12s on 2-3k. those guys all listen to rap and slowed stuff that gets down to the 20s. tuning low will help hit that note but the velocity helps it impact the listener. there are tons of professionals that teach 36hz is plenty low enough b/c of cabin gain. you don't have the cabin gain to make up for frequencies that the port and sub can't play. 

[anr102990]

Yea I was curious about my cabin gain variable in my system. Granted, it’s only a single 12 on an rp 1500...my vehicle is a 2005  supercrew f150 (4door). Basically trying to get the right combination of being able to play low but not ONLY low, and being able to feel that wind, i can definitely feel the wind around tuning within a couple hz above and below. I’m thinking of buying one more DDX 12 and a second rp 1500 amp. My sub is a dual 2 so I can’t put 2 of them on one rp 1500 at .5 ohm, not willing to try it and burn up my amp I paid my hard earned money for lol. 

[Triticum Agricolam]

If you want to ROUGHLY approximate what cabin gain is going to do in WinISD,  you can add a filter like this:

Please note that with the filter enabled it will totally screw up your excursion and port velocity graphs in WinISD.  

[anr102990]

Cool man thanks a lot, I will try that and see how it turns out on the graph. I also have my sub box pushed as far as I can to the passenger side of the rear seat with the port and sub facing up and the port next to the door basically firing up into the rear upwards corner of the cab of my truck. Do you think it’s best to have the sub and port “load” or fire into that corner with it being the furthest point from my ears? Seems like the further away I can get myself from the sub box INSIDE THE TRUCK, the louder it is. The closer I move my head TOWARDS the sub box, it’s not as loud. I also remember several years back when I had my blazer, that it seemed like the vehicle itself amplified the sound of any system VS having the same one in a pickup truck or car trunk..